Alkaline detergents general include alkali metal carbonate and/or hydroxide as the source of alkalinity and are often referred to as ash detergents and caustic detergents, respectively. Detergent formulations employing alkali metal carbonates and/or alkali metal hydroxides are known to provide effective detergency. Formulations can vary greatly in their degree of corrosiveness, acceptance as consumer-friendly and/or environmentally-friendly products, as well as other detergent characteristics. Generally, as the alkalinity of these detergent compositions increase, the difficulty in protecting metal surfaces also increases. A need therefore exists for detergent compositions that minimize and/or eliminate metal corrosion of items within systems employing these detergents.
Various corrosion inhibitors are known and have been used to prevent corrosion of surfaces that come into contact with aqueous alkaline solutions. Some known corrosive inhibitors include the silicates, such as sodium silicate. Unfortunately, the sodium silicates begin to precipitate from aqueous solution at PHS below 11, thus, greatly reducing the effectiveness of these materials to prevent corrosion of the contacted surfaces when used in aqueous cleaning solutions having a lower pH. Additionally, when the silicate-containing compositions or their residues are allowed to dry on the surface to be cleaned, films or spots are often formed, which are visible and which are themselves very difficult to remove. The presence of these silicon-containing deposits can affect the texture of the cleaned surface, the appearance of the surface, and on cooking or storage surfaces, can affect the taste of the materials that come into contact with the cleaned surfaces.
It is also known to include calcium ions within cleaning composition to inhibit the attack of hydroxide ion on alkali sensitive metals. However, it has proven to be difficult to introduce calcium ions into alkaline cleaners without inducing precipitation of hydroxides of the calcium. This is especially true for highly alkaline solutions, such as concentrated solutions that are intended for dilution into use solutions. Theoretically, the protection against corrosion in such systems is based on the presence of the calcium ion in solution, so precipitation of the calcium ions adversely affects the corrosion inhibiting effectiveness of the system. Additionally, the formulations could not include strong chelating agents that could bind with the calcium ion, and again reduce the effectiveness of the calcium ion as a corrosion inhibitor.
Accordingly, it is an objective of the claimed invention to develop alkaline detergent compositions with improved metal protection, reduced precipitation of particulates, and maintained effective detergency.
A further object of the invention is to provide methods for employing alkaline detergents between pHs from about 9 to about 12.5 without causing significant corrosion of metal surfaces.